1. Field of the Invention
The present invention relates generally to the control of biogenic sulfide production. In another aspect, the invention concerns the use of at least one biocide and at least one metabolic inhibitor to synergistically inhibit sulfide production by sulfate-reducing bacteria.
2. Description of the Prior Art
The presence of sulfides (e.g., H2S, HS−, and S2−) in fluids poses serious problems due to their toxicity, odor, and corrosive nature. It is well known that the presence of sulfides in many fluids is a consequence of the reduction of sulfates to sulfides by sulfate-reducing bacteria (SRB). SRB are routinely found in water associated with oil production systems and can be found in virtually all industrial aqueous processes including, for example, cooling-water systems, pulp and paper-making systems, chemical manufacturing, and petroleum refining.
Requirements for SRB activity and growth include a substantially anaerobic aqueous environment containing adequate nutrients, an electron donor, and an electron acceptor. A typical electron acceptor is sulfate, which produces H2S upon reduction. A typical electron donor is a volatile fatty acid (e.g., acetic or propionic acids), although hydrogen can also function as an electron donor. Conditions in an oil reservoir subjected to seawater flooding are excellent for establishing SRB activity. Seawater contains a significant concentration of sulfate, while connate, or indigenous formation, water contains volatile fatty acids and other required trace nutrients (e.g., nitrogen and phosphorus). Conditions within industrial water systems, such as effluent streams from production operations or cooling water streams, are also conducive to SRB activity due to the anaerobic biofilm which is formed on pipeline, tank, or vessel walls. The same is true within the sewers and other piping and facilities associated with municipal wastewater handling systems.
Hydrogen sulfide (H2S) is corrosive and reacts with metal surfaces to form insoluble iron sulfide corrosion products. In oilfield operations, H2S partitions into the water, oil, and natural gas phases of produced fluids and creates a number of problems. For instance, oil and gas that contain high levels of H2S have a lower commercial value than low-sulfide oil and gas. Removing biogenic H2S from sour oil and gas increases the cost of these products. In addition, H2S is an extremely toxic gas and can be lethal to humans at even small concentrations. Its presence in wastewater systems poses a threat to worker safety. The discharge of produced waters containing high levels of H2S into aquatic or marine environments is hazardous because H2S reacts with oxygen and lowers the dissolved-oxygen levels in the water.
Corrosion caused by SRB-produced H2S frequently results in extensive damage. Pipe systems, tank bottoms, and other pieces of equipment can rapidly fail if they have areas where microbial corrosion occurs. If a failure occurs in a pipeline or storage tank bottom, the released fluid can have serious environmental consequences. If a failure occurs in a high pressure water or gas line, the consequences may be worker injury or death. Any such failure involves substantial repair or replacement costs.
In the past there have been two main approaches to reducing the level of sulfides in industrial fluids. One approach involved removing sulfides from the fluids after their formation. This post-formation removal approach, however, was frequently uneconomical or impractical, especially in oilfield operations. The other approach has been to treat the SRB-containing fluids with biocides or metabolic inhibitors to thereby kill or inhibit the growth of the SRB prior to significant biogenic sulfide formation. However, in many instances high concentrations of biocides or metabolic inhibitors are required to effectively inhibit sulfide production by SRB. The costs associated with employing biocides or metabolic inhibitors in such high concentrations can be prohibitive.